Measuring air mass... Is it possible without scales?

[Sane]

Hi everyone, first time here.

 

I have no qualifications in physics what so ever.

 

I am working on an experiment and would like to know if there is a device that can measure the MASS of AIR that is pumped into a container. Does it exist? What is it called?

 

My whole experiment is based on the assumption that such a device exists (I do not want to use scales). I have searched high and low to no avail...

 

If such a device does NOT exist,I have found out that there are AIR FLOW measuring devices (AFM) in the market. Which means I should be able to calculate the air MASS (kg) from the air FLOW (kg/hr).

 

The device I am trying to build pumps air into 2 (air tight) containers until a given pressure is achieved i.e 2 Bars. The second container however, is NOT empty. Assuming that the second container is half full with a fluid or a solid, which of the following is true and WHY?

 

1. The FIRST container will take exactly DOUBLE the TIME to fill than the second one.

OR

2. They will fill up at exactly the same time but the air flow on the FIRST container will be DOUBLE than the air flow on the second.

 

My instinct tells me it's no. 2 but I am not sure. Can you help? Thanks.

[srimukh]

The air pressure will always be under equilibrium at all times i.e, the pressure at every point on the container is going to remain the same. The tube is also to be included under container though. You'll need more (I think the relation is linear) the pressure to pump the same 'mass' of air into the 2nd one than the 1st one. The air flow is more rapid for the 1st container and the both reach the equilibrium at same time.

Edited March 19, 2011 by srimukh

[Schrödinger's hat]

Hi everyone, first time here.

 

I have no qualifications in physics what so ever.

 

I am working on an experiment and would like to know if there is a device that can measure the MASS of AIR that is pumped into a container. Does it exist? What is it called?

 

My whole experiment is based on the assumption that such a device exists (I do not want to use scales). I have searched high and low to no avail...

 

If such a device does NOT exist,I have found out that there are AIR FLOW measuring devices (AFM) in the market. Which means I should be able to calculate the air MASS (kg) from the air FLOW (kg/hr).

 

The device I am trying to build pumps air into 2 (air tight) containers until a given pressure is achieved i.e 2 Bars. The second container however, is NOT empty. Assuming that the second container is half full with a fluid or a solid, which of the following is true and WHY?

 

1. The FIRST container will take exactly DOUBLE the TIME to fill than the second one.

OR

2. They will fill up at exactly the same time but the air flow on the FIRST container will be DOUBLE than the air flow on the second.

 

My instinct tells me it's no. 2 but I am not sure. Can you help? Thanks.

 

If the containers are in equilibrium somehow (ie. they are linked by a pipe, linked to the same hose, or you are only using one pump) then then 2 will be true as the flow will always be such that they are at equal pressure.

Otherwise neither is necessarily true, the flow rate depends on the pressure difference, nozzle size and many factors. Tank 2 will fill up first, but will not take exactly double the time.

 

Depending on exactly what you are trying to do there are some simple and not so simple ways to measure the mass of the air. Any direct mass measurement (ie. using scales or inertia) will be quite cumbersome, but it is not usually too hard to find out how many molecules of whichever compounds are in the tank, and multiply by molecular mass.

Ie. look up or calculate N(T,P,V) somehow where N is number, T is temp, P is Pressure and V is volume.

If you know the volume of the tanks and the pressure you are bringing them to then the simplest way is just to add a temperature monitor to something that is thermally connected to the inside (outside of a metal tank works great, preferably a bit away from the nozzle).

You already have P, and I'm assuming V

The simplest (not terribly accurate, but could probably do the job) model is ideal gas PV=NRT

or N=PV/(RT), multiply by average molecular mass

I encourage you to look up the exact percentages, but something along the lines of 78% N2 21%O2 1% other so 0.8*14grams/mol+0.2*16grams/mol=cbb finding a calculator)

m=PV/RT*14 (in grams)

 

If you need higher precision or help with figuring things out (should find a value for R on wiki, look up ideal gas law to get you started) don't hesitate to ask

[Xittenn]

I don't understand why a pressure gauge is non-sufficient? When one or both hit 2 Bar you have your answer. I mean it takes double the mass to achieve the same pressure in a volume that is doubled in size or is this what you are trying to prove?

 

I am sort of under the impression that you are saying that under the same pressure, gas will flow faster to fill a larger void, in consideration of point number 2. Why would you hypothesize such activity? The only way this could be achieved would be to increase the diameter of the hose connecting your pump to the container, while also maintaining the pressure, to effectively double the throughput.

Edited March 20, 2011 by Xittenn